An optical fiber used for communications includes a core and a cladding disposed thereabout. Considering the fact that the optical fiber may have an outer diameter of 125 microns over the cladding, the connection of two optical fibers such that their cores in the range of about 8 to 50 microns are aligned is a formidable task. Several connectors are available commercially for establishing a connection between optical fibers.
One connector is referred to as a biconic connector. It includes facilities for holding two plugs each of which terminates an optical fiber and each of which has a conically shaped end portion. The optical fiber end terminates in a pedestal which extends beyond an end face of the plug. Two plugs are received in opposite ends of a sleeve which is mounted in a housing. The sleeve includes opposed, conically shaped cavities for receiving the plugs and for holding them in a manner to cause the end faces of the optical fibers to touch or to be spaced apart slightly. The plugs and the sleeves, which are molded, are controlled such that their mating surfaces cause the optical fibers to become aligned when the plugs are received in the sleeve. In this connector, the plugs are free to be turned about their longitudinal axes. Generally, facilities are not provided for repeated alignment of the plugs with respect to the sleeve.
Another connector is referred to as a ferrule type connector. It includes a coupler having a plug-receiving tubular portion at each end thereof. Each tubular portion is provided with a longitudinally extending slot. A sleeve which floats within the coupler is adapted to receive coaxially two plugs each of which is adapted to terminate an optical fiber. Each plug has a passageway extending longitudinally therethrough for receiving an optical fiber and is mounted in a connector body having an alignment pin projecting radially therefrom. When the connector body is received in a tubular portion of the coupler, the alignment pin is received in the slot which extends along the tubular portion. An assembly pin which projects radially from each tubular portion of the coupler is received in a slot of a housing which encloses the associated plug and connector body and which encloses a tubular portion of the coupler when the plug is assembled to the coupler. The slot in the housing includes a circumferential portion in which the assembly pin of the coupler is received to lock the plug to the coupler.
The ferrule type connector is advantageous in that the plugs are made of a ceramic material and are not molded. As a result, the plugs may be machined with close tolerances which is advantageous when dealing with optical fibers having relatively small dimensions. Further, the passageways in the plugs that are destined to receive the optical fibers are made cleanly without the molding flash which may be expected in other kinds of connectors and which could damage the optical fibers.
Although the floating sleeve in the coupler of the ferrule type connector assures that the outer surfaces of the plugs are aligned, it does not insure that the optical fiber cores in the passageways in the plugs are aligned. The plugs may not be disposed concentrically about the optical fibers. Also, the core may be disposed eccentrically with respect to the cladding. Further, misalignment between the cores is caused by the clearance between the optical fibers and the plug passageways in which they are received. Of course, if the fiber cores in the passageways are not aligned, transmission losses ensue. The alignment of the cores is especially critical in connecting single mode fibers wherein the cores typically have a diameter on the order of 8 microns.
The prior art includes a connector system having positioning tabs which assure the same radial orientation of the ends of the optical fibers after repeated couplings. This insures that the fiber ends after disconnecting and reconnecting are always coupled together with the same orientation as before disconnection, but it does not provide for optimal alignment of the optical fiber cores in the first instance.
For field termination of optical fibers, the methods disclosed in commonly assigned application Ser. No. 739,800 filed on May 31, 1985 in the name of J. M. Palmquist are acceptable. Plugs are assembled to connector bodies such that the centroid of the passageway of each plug is aligned radially with the pin of its associated connector body. When optical fibers are terminated with such a plug assembly, the passageways in which the optical fibers are received are aligned with the pins and then with each other when the plugs are received in a coupler. However, for factory termination, more precise methods are sought to insure that the optical fiber cores are aligned.
The need for core alignment has been addressed by the prior art. For example, optimal alignment may be achieved by a trial and error process which involves testing for each iteration of the process. Also, in the biconic connector, referred to hereinbefore, methods and apparatus have been used to abrade the exterior of the biconical plug to remove material therefrom to cause the fiber core of the optical fiber received therein to be disposed concentrically with respect to the exterior surface of the plug. Then, when the plugs are received in a sleeve, the fiber cores are aligned substantially. There is a counterpart of this technique for ferrule type connectors. However, these techniques require precision grinding and generally are expensive.
What is needed and what is not provided by the prior art is a ferrule type system for connecting optical fibers in which provisions are made for causing the optical fiber cores to be aligned optimally. A solution to the problem should be one which is relatively inexpensive to carry out, should be one which is compatible with the presently available ferrule type connectors, and should be one which can be accomplished without iterative testing at the time of installation by the user.